1. Field of the Invention
The present invention relates to a semiconductor device and, more specifically, to a technique to prevent boron implanted to a gate electrode from being diffused into a gate insulator film.
2. Description of the Background Art
A semiconductor device which has a structure including an insulator of an oxide film and metal film formed on a semiconductor substrate is referred to as MOS (Metal Oxide Semiconductor), and a transistor utilizing field effect is referred to as FET (Field Effect Transistor). A transistor having such a structure and utilizing the field effect is referred to as MOSFET, which is adopted into various devices.
The MOSFET has a gate insulator film and a gate electrode stacked on a main surface of a semiconductor substrate, and includes a source region, which is an electron source, and a drain region, which is an outlet of electrons, in the main surface of the semiconductor substrate. A p-channel MOSFET has an n-type semiconductor substrate with source and drain regions of p-type semiconductor. On the contrary, an n-channel MOSFET has a p-type semiconductor substrate with source and drain regions of n-type semiconductor.
A silicon oxide film formed by oxidation of a silicon substrate is commonly used as the gate insulator film, because it can easily be formed and has a good insulation property. A polysilicon film, rather than metal such as aluminum, is commonly used as the gate electrode because of its high thermal resistance. Since pure polysilicon has high electric resistance, it is doped with an impurity. In the p-channel MOSFET, boron is commonly used as the impurity. Doping of the gate electrode with boron is performed by a method to thermally diffuse boron into the gate electrode at high temperature, or by an ion implantation method wherein an ionized impurity is accelerated with an ion accelerator and driven to the object. The ion implantation method is preferred in recent years because of its good controllability. Boron implanted by the ion implantation method or the like is not electrically active, and a crystal structure of silicon becomes irregular due to collisions of ions. Annealing is thus performed to repair the irregular crystal structure of silicon and to electrically activate boron by placing the substrate in inert gas at high temperature.
In this annealing step, boron implanted into the gate electrode sometimes penetrates into the gate insulator film due to thermal diffusion. As a result, a threshold voltage may vary, or a current drive capability may be degraded. One technique to solve this problem is to implant nitrogen ions into the gate electrode prior to the boron implantation to form a nitride film in an end surface of the gate electrode, which end surface will contact with the gate insulator film. As nitrogen is previously implanted into the gate electrode, however, boron does not sufficiently diffuse near the nitride in the gate electrode during the following boron implantation, and a portion with high resistance may remain in the gate electrode. Therefore, there is another solution wherein a silicon nitride film is formed on an upper surface of the gate insulator film, as disclosed in Japanese Patent Laying-Open No. 11-233758.
FIG. 11 shows a cross-section of a general p-channel MOSFET. A source region 20 and a drain region 21 are formed spaced apart with each other in an upper surface of an n-type semiconductor substrate 1. A silicon insulator film 7 as a gate insulator film is formed on a main surface of n-type semiconductor substrate 1 so as to cover at least a portion interposed between the two regions. Silicon insulator film 7 is an oxide film formed by oxidation of a silicon substrate. A boron-doped gate electrode 6 is formed above silicon insulator film 7, and is formed with a polysilicon layer doped with boron. A silicon nitride layer 8 including nitrogen is formed between boron-doped gate electrode 6 and silicon insulator film 7 to prevent boron in boron-doped gate electrode 6 from diffusing to n-type semiconductor substrate 1. Silicon nitride layer 8 is formed by nitriding an upper portion of previously formed silicon insulator film 7.
On the other hand, it is necessary to enhance a current drive capability of a transistor due to higher integration of semiconductor devices in recent years. When a silicon oxide film is used as a gate insulator film, the silicon oxide film is made thinner for this necessity. As the film becomes thinner, its tolerance to penetrated boron decreases. In other words, an acceptable absolute magnitude of boron diffusion into the insulator film decreases because physical thickness of the insulator film decreases.
Because making of the thinner silicon oxide film has a limit, a study is made to use a high permittivity material for a gate insulator film in place of the silicon oxide film. In this specification, the term xe2x80x9chigh permittivityxe2x80x9d means a relative permittivity higher than 3.9, and the material thereof is referred to as xe2x80x9ca high permittivity materialxe2x80x9d. Though the gate insulator film using the high permittivity material is physically thicker than the silicon oxide film, there is still a problem of penetration of boron which was implanted into the gate electrode.
An object of the present invention is to provide a semiconductor device enabling formation of thinner gate insulator film while keeping a tolerance to penetrated boron.
To attain the above-described object, a semiconductor device according to the present invention includes a semiconductor substrate including a source region and a drain region in a main surface thereof, a gate insulator film including a high permittivity material and formed to cover an upper side of a region interposed between the source region and the drain region, a gate electrode formed above the gate insulator film, and a nitride layer formed between the gate insulator film and the gate electrode. With this structure, boron implanted into the gate electrode can be prevented from penetrating into the gate insulator film by the nitride layer, and a variation in a threshold voltage value due to penetrated boron can be inhibited. In addition, the gate insulator film can be made thinner because the high permittivity material is used for the gate insulator film.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.